DC-DC converter

ABSTRACT

A DC-DC converter includes a transformer and a switching element. The primary winding of the transformer and the switching element are connecting in series with a DC power source. A DC output is extracted via a rectifier circuit connected to a secondary winding of the transformer. A capacitor is connected in parallel to the switching element, and a saturable reactor is connected between the secondary winding of the transformer and the rectifier.

The present invention relates to the structure of a DC-DC converter designed to reduce losses.

BACKGROUND OF THE INVENTION Description of the Prior Art

Recent developments in switching elements that can be used at high frequencies have resulted in an increase in the switching frequencies of DC-DC converters, and this in turn has led to expectations of reductions in the size of DC-DC converters because it should now be possible to construct smaller versions of the transformers, choke coils, and smoothing capacitors that take up so much room conventionally.

However, the switching losses that occur as current and voltage are superimposed whenever switching elements turn on and off increase as frequencies increase. This means that, regardless of how small these components and other circuit elements become, at present the heat dissipation countermeasures necessary for coping with the heat generated by such losses ensure that sizes cannot be reduced further.

A circuit diagram of a conventional forward DC-DC converter with one switching transistor is shown in FIG. 2, and the timings of voltage and current waveforms in this DC-DC converter are shown in FIG. 4.

In this DC-DC converter, a DC source E_(s), a primary winding L₁ of a transformer T₁, and a transistor Q₁ that acts as a switching element form a series circuit, and a rectifying and smoothing circuit formed of a rectifier diode D₁, a choke coil L₃, a fly-wheel diode D₂, and a smoothing capacitor C₁ is connected to a secondary winding L₂ of the transformer T₁.

The transistor Q₁ receives a gate voltage from a control circuit that is not shown in the figure.

While the transistor Q₁ of the above DC-DC converter is on, a current flows through the primary winding L₁ on the input side of the transformer T₁, and a DC output is obtained at output terminals 1 and 1' from a voltage induced in the secondary winding L₂ on the output side, using the rectifying and smoothing circuit.

FIG. 4 shows the waveforms of the gate voltage V_(G1) of the transistor Q₁, the drain-source voltage V_(Q1) of the transistor Q₁, and the current I_(Q1) flowing through the transistor Q₁ via the primary winding L₁, expressed against the same horizontal time axis. As can be seen from the figure, the drain-source voltage V_(Q1) and the current I_(Q1) are superimposed during a period between a time t₁ at which the transistor Q₁ turns on and a subsequent time t₂, and during a period between a time t₃ at which the transistor Q₁ turns off and a subsequent time t₄. This superimposition causes switching losses.

In the above conventional DC-DC converter, the switching losses that occur as described above increase as frequencies increase. In addition, if an insulated-gate field-effect transistor is used as the transistor Q₁ that acts as the switching element, a parasitic capacitor C₂ that is parasitic on such a transistor is in a charged condition at the time t₁ at which the transistor turns on, and both power is lost and noise is generated by the consequent shorting of the parasitic capacitor C₂.

SUMMARY OF THE INVENTION Purpose of the Present Invention

The object of the present invention is to provide a DC-DC converter with one switching transistor designed to reduce losses.

Mode of Use of the Present Invention

The present invention relates to a DC-DC converter comprising a DC power source, a primary winding of a transformer, and a switching element connected in series, whereby a DC output is extracted via a rectifier and smoothing circuit connected to a secondary winding of the transformer while the switching element is on. The DC-DC converter is characterized in that a capacitor is connected in parallel to the switching element, and a saturable reactor is connected between the secondary winding of the transformer and the rectifier diode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an embodiment of the DC-DC converter of the present invention,

FIG. 2 is a circuit diagram of a conventional DC-DC converter,

FIG. 3 is a timing chart of currents and voltages in the DC-DC converter of the present invention, and FIG. 4 is a timing chart of currents and voltages in the DC-DC converter of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the DC-DC converter of the present invention will be described below with reference to FIG. 1. Elements in FIG. 1 that are the same as those in FIG. 2 are denoted by the same reference notation.

The DC-DC converter with one switching transistor of FIG. 1 comprises a series circuit of a DC source E_(S), a primary winding L₁ of a transformer T₁, and an insulated-gate field-effect transistor Q₁ that acts as a switching element, with a rectifying and smoothing circuit formed of a rectifier diode D₁, a choke coil L₃, a fly-wheel diode D₂, and a smoothing capacitor C₁ being connected to a secondary winding L₂ of the transformer T₁.

In addition, a capacitor C₃ is connected in parallel to the transistor Q₁, a saturable reactor L₄ is connected between the rectifier diode D₁ of the rectifying circuit and the secondary winding L₂, and electric power is transferred via the transformer T₁ while the transistor Q₁ is on, transistor Q₁ being applied a gate voltage from a signal generating circuit 2.

The operation of the DC-DC converter of the above construction will now be described with reference to the timing chart of FIG. 3.

FIG. 3 shows the waveforms of a gate voltage V_(G1) and a drain-source voltage V_(Q1) of the transistor Q₁, a current I_(Q1) flowing from the drain to the source of the transistor Q₁, a current I_(D1) flowing through the rectifier diode D₁, a current I_(D23) flowing through the fly-wheel diode D₂, and a current I_(C23) that is the sum of currents flowing through the parasitic capacitor C₂ of the transistor Q₁ and the capacitor C₃, expressed against the same horizontal time axis.

First, at a time t₅ at which an on period ends, the gate voltage V_(G1) of the transistor Q₁ becomes zero and the transistor Q₁ turns off, whereupon the current I_(Q1) that was flowing from the primary winding L₁ of the transformer T₁ to the transistor Q₁ becomes zero. However, magnetizing current that was flowing through primary winding L₁ during on period tends to flow continuously, owing to the reaction of primary winding L₁. For this reason, this current flow through the parasitic capacitor C₂ of the transistor Q₁ and the capacitor C₃ connected parallel to the transistor Q₁, and is figured I_(C23) (the sum of the currents flowing through the two capacitors) in FIG. 3. Therefore, the voltage between the two ends of the parasitic capacitor C₂ and the capacitor C₃ (in other words, the drain-source voltage V_(Q1) of the transistor Q₁) rises smoothly while charging the parasitic capacitor C₂ and the capacitor C₃.

Therefore, no switching losses are generated by the drain-source voltage V_(Q1) and the current I_(Q1) being superimposed when the transistor Q₁ is turned off.

Subsequently, starting at a time t₆ at which the voltage V_(Q1) exceeds the voltage V_(ES) of the DC source E_(S), a resonance phenomenon is generated in the series circuit consisting of the primary winding L₁, the combined capacitance of the parasitic capacitor C₂ and the capacitor C₃, and the DC source E_(S).

This results in a voltage being applied to the primary winding L₁ of the transformer T₁ in the direction Opposite to that when the transistor Q₁ is on, after the time t₆ at which the voltage V_(Q1) exceeds the voltage V_(ES) of the DC source E_(s). Therefore the voltage between the two ends of the secondary winding L₂ becomes negative at the polarity point side marked by the dot (·) in FIG. 1, the current I_(D1) flowing through the rectifier diode D₁ disappears and the rectifier diode D₁ turns off, and therefore the output current flows as a current I_(D2) through the diode D₂ and the secondary side of the transformer T₁ is opened in result.

The frequency f of this resonance is given by Equation (1), and the voltage between the ends of the capacitor C₃ (in other words, the drain-source voltage V_(Q1) of the transistor Q₁) changes corresponding to a sine waveform of resonance. ##EQU1##

In Equation (1), L₁ is the inductance of the primary winding L₁, C₂ is the parasitic capacitance of the transistor Q₁, and C₃ is the capacitance of the capacitor C₃.

After the drain-source voltage V_(Q1) reaches its peak, it falls smoothly but, immediately after a time t₇ at which it falls below the voltage V_(ES) of the DC source E_(S), a voltage is again applied to the primary winding L₁ in the same direction as that when the transistor Q₁ is on.

As a result, the polarity point side of the secondary winding L₂ marked by the dot (·) in FIG. 1 becomes positive, and, if the rectifier diode D₁ is connected directly to the secondary winding L₂, the rectifier diode D₁ becomes conductive at the same time as the diode D₂ which has been conductive up to this point, and the secondary winding L₂ of the transformer T₁ is electrically short. For this reason, there is no longer any inductance in the primary winding L₁ and hence there is no longer a resonant component, so the resonance does not continue.

However, in the Dc-DC converter of the present invention, the saturable reactor L₄ is connected between the secondary winding L₂ and the rectifier diode D₁, and the time at which the rectifier diode D₁ becomes conductive is delayed during the period is delayed during the period between the time when the polarity side of the secondary winding L₂ marked by the dot (·) in FIG. 1 became positive (the time t₇) and the time when saturable reactor L₄ is saturated. During this period, the secondary winding L₂ of the transformer T₁ is still electrically opened, so the resonance continues and, at a time t₈, the voltage between the ends of the capacitor C₃ (in other words, the drain-source voltage V_(Q1)) becomes zero. Of course, the period said above must be made longer than the period between the time t₇ and the time t₈.

The control circuit operates to turn on the transistor Q₁ at the time t₈ at which it detects that the voltage V_(Q1) has become zero. Note that the circuit that detects the state of the voltage V_(Q1) in this manner is not shown in FIG. 1.

Since the transistor Q₁ turns on after the voltage V_(Q1) becomes zero, the voltage V_(Q1) and the current I_(Q1) are not superimposed when it turns on at time t₈, so no switching losses that would be caused by such superimposition occur.

In addition, before the transistor Q₁ turning on, resonance causes the drain-source voltage V_(Q1) (in other words, the voltage between the ends of the parasitic capacitor C₂ and the capacitor C₃) to fall to zero, so no losses or noise caused by shorting of the parasitic capacitor C₂ are generated.

Note that in FIG. 3, the current I_(C23) of the parasitic capacitor C₂ of the transistor Q₁ and the capacitor C₃ is denoted by (+) when it flows from the primary winding L₁ through the parasitic capacitor C₂ and the capacitor C₃, and by (-) when it flows in the opposite direction.

In the above structure, no switching losses are generated when the transistor turns either off or on, and no losses or short-circuit noise caused by the parasitic capacitor C₂ are generated.

It should be noted that the switching element could also be a bipolar transistor, and that the present invention can be applied to a wide variety of forward DC-DC converters, each having one switching transistor.

Effect of the Present Invention

As described above, in the DC-DC converter of the present invention, a capacitor is connected in parallel to the switching element, and a saturable reactor is connected between the secondary winding of the transformer and the rectifier diode.

In addition, when the switching element turns off, the magnetizing current flows through the capacitor connected in parallel to the switching element, ensuring that the voltage between the two ends of the switching element rises smoothly. When the switching element turns on, a resonance phenomenon continues, causing the voltage between the ends of the switching element to fall to zero.

In this way, there is no period during which voltage and current are superimposed, both when the switching element turns off and when it turns on, hence there are no switching losses. 

What is claimed is:
 1. A DC-DC converter comprising:a transformer having a primary winding and a secondary winding, said primary winding being adapted to be connected in series with a DC power source; a rectifier and smoothing circuit; a switching means, adapted to be connected in series with said DC power source, for connecting said rectifier and smoothing circuit to said secondary winding of said transformer during a time when said switching means is in an on-state; a capacitor connected in parallel with said switching means; and a saturable reactor connected between said secondary winding of said transformer and said rectifier.
 2. A DC-DC converter according to claim 1, wherein said switching element is an insulated-gate field-effect transistor. 